Specific heat study of doping effects in α'-NaV2O5

In the low dimensional spin System α'-NaV2O5, a charge ordering phase transition and the formation of a spin-singlet state are almost degenerate at T0 congruent with 34 K. To foster our understanding of the interplay between charge and magnetic degrees of freedom, we have investigated the influence of chemical substitution on the phase transition by measuring the specific heat of doped single crystals. Significant changes have been observed already at low substitution levels. While isoelectrical substitution (Li, K) leads to a uniform decrease of T0 and of the size of the anomaly Δcp at T0, non-isoelectrical substitution (Ca) leads to a strong reduction of Δcp but only to a weak decrease of T0. Once the phase transition has disappeared, a linear term and an enhanced cubic term βd appears in cp between 2 and 19 K, cp=γT+βdT3, which we attribute to magnetic excitation of the effective S=1/2 spin chain and to valence fluctuation, respectively